Abstract

Chilli CM334 (Capsicum annuum L.) is resistant to Phytophthora capsici Leonian (Pc), but Nacobbus aberrans Thorne and Allen, 1944 (Na) broke down its resistance in plants previously infected by the nematode. Peroxidase (POD) and L-phenylalanine ammonia-lyase (PAL) activity, total soluble phenols (TSP) and chlorogenic acid concentration in CM334 plants inoculated with either or both pathogens (Na-Pc) were compared; also, the toxic effect of some phenolic acids on Na was tested in vitro. The highest POD activity (5.3 μM tetraguaiacol mg−1 protein min−1) was registered in plants inoculated only with Pc, while those inoculated only with Na showed the lowest (3.3 μM) (P ≤ 0.05). PAL activity was 39.9 nM trans-cinnamic acid μg−1 protein min−1 in plants inoculated only with Pc, and it was lower (19.3 nM) and similar in non-inoculated plants or those with Na and with Na-Pc (P ≤ 0.05). Usually, plants inoculated with Pc alone had higher contents of TSP (P ≤ 0.05) (1.9 mg tannic acid g−1 dry matter) and plants inoculated with Na or Na-Pc had lower levels (0.8 and 0.9 mg) than those non-inoculated (1.3 mg). CM334 plants inoculated with Na showed a significant reduction (10–37% and 12–17%, in roots and leaves) in the concentration of chlorogenic acid as compared to the non-inoculated. Vanillic, trans-cinnamic, p-coumaric and syringic acids had greater nematicidal effects (P ≤ 0.05) than chlorogenic acid in vitro. Apparently Na modified the defence responses in CM334 plants as POD and PAL activities and TSP and chlorogenic acid concentrations were reduced.